Named group helps on readability and make the regexp "self-documenting". In other user cases this would also keep the code more flexible in case of small variation in the output, you receive a provider name before the phone number.

The phone number parsing stays the same, as the code doesn't depend on the position/index, you just add a provided named group to extract the provider.

Closing words

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.
- Jamie Zawinski

First regular expressions are powerful, their simplicity hides often complex processing. Solving your text parsing/validation problems in one line is great but this line needs way much more testing than the other lines in your codebase. You don’t have a PhD in Regular Expression (and most of your colleague either), so don’t leave them blind when they’ll have to adapt your regexp. Leave a few unit tests to show what is an expected valid/invalid content.

Second "when you have a hammer, everything looks like a nail", most of the time, you can drop your homemade expression and favor a well tested implementation (eg File.extname).